An image pick-up device which illuminates an object with a light having a known spectral distribution and receives light reflected by the object to obtain an image of the object (for example, JP3214195B) and a distance measuring device which measures a distance to an object (JP3272699B) have been developed. Further, an object detecting device which emits signal light having a known spectral distribution in the wavelength range of infrared light and the like toward the space to determine whether or not an object is present in the space is widely used.
When such an image pick-up device, such a distance measuring device, such an object detecting device and the like are used outdoors, in particular, accuracy of distance measurement and object detection will be significantly affected by ambient light such as the sunlight. In order to reduce the influence of the ambient light, a band-pass filter which passes through light in a wavelength range of the signal light is used. However, when intensity of the sunlight has a very strong influence outdoors, ambient light cannot be sufficiently removed by conventional methods.